272 CEPHALOPODA. 



Fig. 127, for instance, resembles the adult more than do embryos of 

 LoKgo and Sepia at somewhat corresponding stages (Fig. 119, p. 261, 

 and Fig. 129 C, p. 275). By the further growth of the mantle, 

 the appearance on it of fins, as well as the closing of the funnel which 

 is grown over by the mantle and, like the gills, partly enclosed in 

 the mantle-cavity, the embryo approaches the adult form more and 

 more. The eyes which, in this embryo, are very large, also decrease 

 in size, as their internal structure gradually develops. The arms 

 grow and become covered with suckers. The swelling between them 

 (the external yolk-sac) gradually disappears, and the internal yolk- 

 sac, which here also tills a large part of the embryo, is gradually 

 I nit very slowly absorbed. The oral aperture, in consequence of this 

 process of growth, shifts downward to between the arms, to take up 

 its final position at the anterior end of the body. At about this 

 stage, the embryo breaks through the egg-envelopes and, during tin' 

 period of pelagic life which now ensues, assumes the final form of 

 the adult. 



C. Development through the formation of the Embryonic Rudi 



ment on a limited portion of the egg with simultaneous 



development of a large Yolk-sac. 



The type of development now to be considered has only been 

 observed in the egg of Sepia (Kolliker, No. 24; Vialleton, No. 

 48), but may occur in other specially large Cephalopodan eggs as 

 well. This form of development is brought about by the abundance 

 of yolk in the egg, and is characterised by the restriction of the 

 embryonic rudiment to a small, cap-like portion of the egg (the germ- 

 disc), the rest of the yolk being enclosed merely by the thin cell- 

 integuments already mentioned (ectoderm, mesoderm and entoderm, 

 p. 257). There is a further difference between this form of develop- 

 ment and that in which the embryonic rudiment extends over a 

 considerable part of the egg, inasmuch as the embryo appeals on a 

 surface only slightly arched (Fig. 128 J and />'). This makes the 

 processes of development somewhat more difficult to understand, but 

 we are assisted in following them by comparison with the same 

 processes in eggs less rich in yolk. 



The first indications of the embryo on the germ-disc take the form 

 of various prominences and swelling's which soon show bilateral 

 symmetry in their shape and arrangement. Tins symmetry most 

 probably corresponds to that already evident in the germ-disc during 



